Since they have everything, send them a fruitcake in a tin. Darn things last forever. When they need the fruitcake, they can open the tin, throw out the cake and use the tin for water storage, a shovel, a brazier, candle holder, a small game trap, ,hat,ammo storage, water cup, food storage, mixing bowl, etc.
And best of all, its less than 50 bucks so you can buy more than one.
Heh, some of that B&M canned bread (check Kroger) might do better than fruitcake.
As for reusable gifts, check out the BlastMatch fire starter. $30 or so, but dead simple to use. Do they have backpacks? If not, a milsurp ALICE pack can be had pretty cheap.